As well as jailing him, Judge Peter Davies made Kumah the subject of a sexual prevention order for 10 years, saying that this and his previous convictions for two offences of indecent assault against women in 1996 make him a “serious risk of harm”.

The victim, now aged 16, who cannot be named for legal reasons, was a passenger on the bus from Bolton to Little Hulton when she was approached by Kumah on the top deck.

Judge Davies said: “You engaged in sexual activity in front of a 15-year-old girl. Anybody who saw the DVD will clearly see she is a young and vulnerable teenager.

“You followed her, she was sat on the top deck of the bus. You sat down across the aisle and exposed yourself.

“You kept staring at her and asked if she liked it. She was dumbfounded.

"It was depraved, it was corrupt. She was horrified and ran downstairs and sought comfort from a fellow female passenger. You alighted and made your way home.

“The identification evidence was compelling. You forged computer documents in a bid to deceive the jury, saying that you were working on an office computer.